Early IMDs such as implantable cardiac pacemakers were designed to operate in a typically single operating mode governed by fixed operating parameters without any ability to change the operating mode or otherwise communicate percutaneously with external equipment. In time, it became apparent that it would be clinically desirable to vary certain of the operating parameters and/or modes of operation. An initial approach employed with implanted cardiac pacemakers involved use of miniature rheostats that could be directly accessed by a needle-like tool inserted through the patient's skin to adjust a resistance in the pacing rate or pulse width setting circuit. Later, miniaturized reed switches were incorporated into the pacing rate or pulse width circuits that responded to magnetic fields applied through the skin by an external magnet placed over the implant site. The pulse width, pacing rate and a limited number of pacing modes could be adjusted in this manner.
It was also realized that the operation of an implantable cardiac pacemaker could be observed, for example, by use of a standard EKG machine and timing of intervals between pacing pulse spikes in the ECG tracing recorded from skin electrodes on the patient. The applied magnet was used to close a reed switch to change the pacing mode to an asynchronous pacing mode and to encode the fixed pacing rate or pulse amplitude or width to a value reflecting a current operating parameter. One use of this technique was to monitor impending battery depletion through observation of a change in the pacing rate from a preset or programmed pacing rate in response to a battery voltage drop, as described, for example, in U.S. Pat. No. 4,445,512. This approach could only provide a low bandpass data channel, of course, to avoid interfering with the primary function of pacing the patient's heart when necessary.
As digital circuit technology advanced, it was recognized that control of operating modes and parameters of implanted medical devices could be realized in digital or binary circuits employing memorized control states or operating parameter values. In order to change an operating mode or parameter value, "programmers" were developed based on radio frequency (RF) downlink data communication from an external programmer transceiver to a telemetry transceiver and memory incorporated within the IMD.
Through the use of such telemetry systems, it became possible to provide uplink data telemetry to transmit the contents of a register or memory within the IMD to the telemetry receiver within the programmer employing the same RF transmission capabilities. Today, both analog and digital data can be transmitted by uplink RF telemetry from the implanted medical device to the external programmer. In the context of implantable cardiac pacemakers, the analog data typically includes battery status, sampled intracardiac electrocardiogram amplitude values, sensor output signals, pacing pulse amplitude, energy, and pulse width, and pacing lead impedance. The digital data typically includes statistics related to performance, event markers, current values of programmable parameters, implant data, and patient and IMD identification codes.
The telemetry transmission system that evolved into current common use relies upon the generation of low amplitude magnetic fields by current oscillating in an LC circuit of an RF telemetry antenna in a transmitting mode and the sensing of currents induced a closely spaced RF telemetry antenna in a receiving mode. Short duration bursts of the carrier frequency are transmitted in a variety of telemetry transmission formats. In the MEDTRONIC.RTM. product line, the RF carrier frequency is set at 175 kHz, and the RF telemetry antenna of the IMD is typically coiled wire wound about a ferrite core that is located within the hermetically sealed enclosure. The RF telemetry antenna of the external programmer is contained in a programming head together with a permanent magnet that can be placed on the patient's skin over the IMD to establish a magnetic field within the hermetically sealed enclosure of the IMD.
In an uplink telemetry transmission from an implanted medical device, it is desirable to limit the current drain from the implanted battery as much as possible simply to prolong device longevity. However, as device operating and monitoring capabilities multiply, it is desirable to be able to transmit out ever increasing volumes of data in real time or in as short a transmission time as possible with high reliability and immunity to spurious noise. As a result of these considerations, many RF telemetry transmission data encoding schemes have been proposed or currently are used that attempt to increase the data transmission rate.
At present, a wide variety of IMDs are commercially released or proposed for clinical implantation that are programmable in a variety of operating modes and are interrogable using RF telemetry transmissions. Such medical devices include implantable cardiac pacemakers, cardioverter/defibrillators, pacemaker/cardioverter/defibrillators, drug delivery systems, cardiomyostimulators, cardiac and other physiologic monitors, electrical stimulators including nerve and muscle stimulators, deep brain stimulators, and cochlear implants, and heart assist devices or pumps, etc. As the technology advances, IMDs become ever more complex in possible programmable operating modes, menus of available operating parameters, and capabilities of monitoring increasing varieties of physiologic conditions and electrical signals. These complexities place ever increasing demands on the programming and interrogation system and the medical care providers using them.
In our Statutory Invention Registration H1347, we disclose an improvement to programmers of this type adding audio voiced statements that accompany their operations to assist the medical care provider using them. For example, we propose adding voiced statements that track interactive operation of a programmer and implanted medical device during programming and patient follow-up sessions that can be heard by the medical care provider using the programmer. Such voiced statements would augment or replace the visual display of such information or minimal audible tones (e.g., beeps) that are displayed or emitted in use of the external programmer or pacing system analyzer.
Other approaches than reliance upon RF telemetry transmissions have also been developed for providing real time warnings to the patient that the IMD is malfunctioning or is about to deliver a therapy in response to a detected need. Audible beeping alarms have been proposed to be incorporated into the IMD to warn the patient of battery depletion as disclosed for example in U.S. Pat. Nos. 4,345,603 and 4,488,555, incorporated herein by reference. Similarly, the application of low energy stimulation to electrodes on or near the IMD to "tingle" the patient upon battery depletion have been proposed in U.S. Pat. Nos. 4,140,131, and 5,076,272, incorporated herein by reference, and also in the above-incorporated '603 patent. Use of the audible beeping alarm incorporated into an implantable cardioverter/defibrillator to warn the patient of impending delivery of a cardioversion shock is disclosed for example in U.S. Pat. No. 4,210,149, incorporated herein by reference.
Moreover, it has been proposed to employ acoustic beeping warnings of implantable cardiac pacemaker battery depletion in an external monitor which apparently is directly coupled with an implanted cardiac pacemaker in U.S. Pat. No. 4,102,346. Acoustic voice recordings have been incorporated into external medical devices to provide warnings or instructions for use as disclosed in U.S. Pat. Nos. 5,285,792, 4,832,033, and 5,573,506, incorporated herein by reference.
As noted above, the historical development of IMDs has been marked by ever increasing sophistication and complexity in design and operation. However, in certain circumstances, it is desirable to provide simplified IMDs having limited features and controllable operating modes and parameters for use in developing countries or that can be controlled by the patient.
As an example of the former case, a simplified and low cost programmable, single chamber cardiac pacemaker pulse generator is disclosed in commonly assigned U.S. Pat. Nos. 5,391,188 and 5,292,342, incorporated herein by reference, specifically intended to meet demand in emerging countries. In order to avoid the need for expensive external programmers, the low cost pacemaker disclosed therein is designed to employ a simplified programming scheme and a simple EKG display coupled to skin contact electrodes for simply displaying the pacing pulse artifact and patient's ECG. In this low cost implantable cardiac pacemaker, programming is effected by repeated timed applications of the magnetic field to the IMD as described therein to incrementally increase or decrease pacing rate, pacing pulse width amplitude, etc. The magnetic field can be manually applied and removed and the field polarity can be reversed. A magnetic field sensor and associated programming circuitry within the IMD responds to the application and polarity of the magnetic field to make the incremental changes. The medical care provider must closely watch the EKG display and calculate the changes in pacing rate from the observed changes in pacing interval and scale changes in pulse amplitude. This requires good hand-eye coordination and rapid mental calculation to determine just when the desired rate or amplitude change has been accomplished.
In the latter case, neurological stimulation devices and drug delivery systems are available for implantation in a patients' body, and external programmers for providing limited adjustment of stimulation therapies and delivery of drugs are provided to the patients to allow them to adjust the delivered therapy. Such devices include the MEDTRONIC.RTM. Itrel.RTM. implantable nerve stimulator and Synchromed.RTM. drug infusion system. The patients are allowed to adjust the stimulation and drug therapies by transmitting "increase" and "decrease" commands. The implanted medical device responds to the programmed command, but the response is not communicated back to the patient who may remain concerned that the desired adjustment has not been made.
All of the abovedescribed RF telemetry systems require complex circuitry, and bulky antennas as described above and are expensive to implement into an IMD. The RF telemetry transceiver in the IMD consumes electrical energy from the device battery while it is in use. Moreover, the telemetry systems all require use of an expensive and complex external programmer that establishes the telemetry protocol, encodes and transmits the downlink telemetry transmissions, and receives, decodes and displays and/or records the uplink telemetry transmissions. The uplink telemetered data from the IMD and device operations, e.g., delivery of pacing pulses by an implantable cardiac pacemaker, are recorded and/or displayed only visually, requiring careful visual observation by the medical care provider operating the programmer. Similarly, confirmation of acceptance of a programmed change in an operating mode or parameter value can only be observed using a recorder or visible display of a confirmation received in an uplink telemetry transmission. It is desirable to provide a simple way to confirm the acceptance of a programmed change that would not require use of RF telemetry equipment by the patient or medical care provider and RF telemetry capability in the IMD. This is especially the case in programming limited operating modes and parameter values by a patient using a limited function programmer. As will become apparent from the following, the present invention satisfies many of these needs.